What is the diameter of the hose / pipe /tube (right word) ?
IIRC it is possible to measure flow by changes in electric field due to flow (sorry no link, it will probably depend on the liquid)
the sampling rate is at least 10 Hz to give me a useful result
that means samples of 0.3 ml??
According to wikipedia: one drop of water is ~ 50 microliter.
So were talking about counting 3 ml/sec = 60 drops per second Think that is too fast to make some drop counting device, 60 drops is a continuous jet allready I think.
If it are still drops maybe a laserbundle on a photocell?
What is the range of flow that must be measured?
What is an acceptable error?